1. Field of the invention
The present invention relates an apparatus for recording, reproducing, or transmitting a digitized component video signal, and specifically to a method of positioning a digital component video signal to the recording medium, and to a recording apparatus achieving such positioning, in a digital video cassette recorder or similar apparatus.
2. Description of the prior art
Digitization of component video signals currently conforms to CCIR Rec. 601, "Encoding parameters of digital television for studios." Based on this recommendation, the sampling frequency for the luminance signal is 13.5 MHz, the sampling frequency for the color difference signals is 6.75 MHz, and both luminance and color difference signals are quantized at 10 bits/sample. The valid sampling count in one horizontal scanning period is 720 samples for the luminance signal Y and 360 samples each for the two color difference signals Cb, Cr. Current television signals with a 4:3 aspect ratio and scanning format of either 525 lines at 60 Hz ("525/60 format" below) or 625 lines at 50 Hz ("625/50 format" below) are digitized according to this CCIR recommendation. The structure of the sampling points in this recommendation is shown in FIG. 2.
As shown in FIG. 2, points that are simultaneously sampled for the luminance signal Y and the color difference signals Cb, Cr alternate in each line with points sampled only for the luminance signal Y. In FIG. 2, for example, both the luminance signal Y and the color difference signals Cb, Cr are sampled for pixel 201, but only the luminance signal Y is sampled for pixel 202.
The so-called D-1 standard is used for conventional video tape recorders capable of recording/reproducing component video signals sampled according to CCIR Rec. 601. According to this D-1 standard, however, the data actually recorded to/reproduced from the video tape is only the high eight bits including the MSB of the 10-bit quantized data.
This D-1 standard is described in detail in "SMPTE 227M 19-mm type D-1 cassette-helical data and control records," SMPTE Journal, March 1992.
The tape pattern recorded in this D-1 digital VCR is shown in FIG. 17. In the D-1 standard, one field of each 525/60-format television image is divided into five segments, one field of each 625/50-format television image is divided into six segments, and signal processing is completed for data blocks of 50 horizontal scanning periods. The data for each segment is divided into four parts and distributed to four recording channels. The video data for one segment is recorded on four sectors, and thus four sectors 2, 3, 4, and 5 shown in FIG. 17 form one segment.
As shown in FIG. 17, these four video sectors 2, 3, 4, and 5 are located in four different tracks, and are therefore recorded/reproduced by four different heads. In other words, the video data distributed to each of four recording channels each forms one video sector, each of which is recorded/reproduced by a different head.
Distribution of the video data to four recording channels occurs each time the Y, Cb, and Cr components are sampled. This is illustrated in FIG. 18. This figure represents one sampled part of the complete television screen; for these three consecutive lines, the value shown for each Y, Cb, and Cr component sampling point indicates the number of the channel to which the sample is recorded.
As shown in FIG. 18, for each of the Y, Cb, and Cr component samples, the eight samples surrounding one sampling point (i.e., the sampling points above and below, right and left, and diagonally to the center sampling point; right and left sampling points taken at one sampling point intervals for the Y component, and two sampling point intervals for the Cb and Cr components) are always recorded to a recording channel different from that of the center sampling point. This is to enable image concealment when head clogging occurs with the objective being a reproduced image free of any visible image defects.
Head clogging as used herein occurs when one of the recording or reproducing heads functions abnormally either temporarily or permanently during recording or reproducing, and all data recorded by the recording head or reproduced by the reproducing head is in error. As a result, the number of error samples makes error correction using the error correction code impossible when head clogging occurs, and error detection only occurs during decoding of the error correction code. Detected error samples are therefore processed by means of error concealment so that they do not stand out from the surrounding image.
In error concealment, error samples detected by the error correction code are replaced with data obtained by selective interpolation filtering of the surrounding error-free pixel data (data from vertically, horizontally, or diagonally adjacent pixels) and processing the data so it does not stand out. It is therefore necessary for the samples surrounding the error sample to be as error-free as possible if image correction by error concealment is to function effectively.
Channel distribution in the D-1 standard is designed to assure this. As shown in FIG. 18, the surrounding eight samples of the discrete Y, Cb, and Cr components are recorded to a recording channel different from that of the recording channel of the error sample, thereby enabling recording/reproducing using a head free of head clogging when error samples from head clogging are detected.
As also shown in FIG. 18, however, the Cb and Cr component samples of the same pixel are always distributed to the same channel, and are therefore recorded by the same head. The Y component sample of the same pixel, however, is not necessarily recorded by the same head. Y samples recorded by the same head may be up to two horizontal pixels removed from the Cb and Cr component samples.
As a result, pixels for which only the Y component sample is in error and pixels for which only the Cb and Cr component samples are in error occur when head clogging occurs. Because a display pixel can only be generated when all three (Y, Cb, and Cr) components are available, pixels for which only the Y component sample is in error and pixels for which only the Cb and Cr component samples are in error must be handled as though a pixel error was detected.
The problem with this D-1 recording method, therefore, is that the number of display pixels containing errors and displayed by means of error concealment increases because the total number of pixel errors in the full screen image increases.